1. Field of the Invention
The present invention relates to an image forming apparatus which can change the magnification of image information when printing the image on a printing medium in the process of electrophotography.
2. Description of the Related Art
There is a conventional image forming apparatus, such as a copier and a printer, which can modify the magnification when printing in the process of electrophotography. The magnification of such image forming apparatus is adjusted by an operator by adjusting the rotation speeds of a plurality of motors, such as a polygon motor and a transfer motor, on shipment or field service. For example, an operator adjusts the speed of a polygon motor when changing the sidewise magnification or the main scanning direction, and adjusts the speeds of a transfer motor, a drum motor, a registration motor and a fixing motor when changing the lengthwise magnification or the subsidiary scanning direction. These magnification adjustments are completed by repeating the adjustment of each motor speed to finally obtain the correct magnification, i.e., adjusting one motor after another, and adjusting the other motor again including the first adjusted one after adjusting one motor.
However, this way of magnification adjustment executed by an operator by repeating the motor speed adjustment causes fluctuation in the time and labor taken for the adjustment, and depends on the operator's skill. Further, when adjusting the magnification in the subsidiary scanning direction, the operator changes the speed setting of a transfer motor, for example, and the paper feed speed changes when passing through a transfer unit. The paper feed speed change when passing through a transfer unit causes a shift between the paper carry speed by the motor of a carry system used for feeding the paper, causing a paper jam in the paper carrying path.
Therefore, there is a need for an image forming apparatus which can easily adjust the magnification, and prevent a paper jam in the paper carrying path, at the same time.